Binderless panel made from wood particles and cellulosic fibers

ABSTRACT

The present invention relates to pressed fiberboard articles made using a binderless wet process from a combination of fibers and particles. A mixture of particles (e.g., wood particles) and cellulosic fibers is formed into a panel using only water, heat and pressure. The present invention is further directed to fiberboard articles having uniform density cores or higher density cores.

The present application claims priority to U.S. Provisional Application Ser. No. 61/594,921, filed Feb. 3, 2012, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to pressed fiberboard articles made using a binderless wet process from a combination of fibers and particles. A mixture of particles (e.g., wood particles) and cellulosic fibers is formed into a panel using only water, heat and pressure. The present invention is further directed to fiberboard articles having uniform density cores or higher density cores.

BACKGROUND OF THE INVENTION

Wood-based composites and pressed wood products are engineered sheet goods made from wood elements held together by an adhesive bond. Examples of commercial wood-based composites include such products as S2S (smooth 2 sides) hardboard, medium density fiberboard (MDF), particleboard (PB), and oriented strand board (OSB). The type of wood elements, adhesive(s), and the panel-forming method used to make a panel determines the final composite product and its performance. The essential steps of manufacturing can be reduced to: a) the generation of wood elements and b) the consolidation of those wood elements into sheet form.

Wood or cellulosic elements used in composite formation range in size and shape from smaller elements, like fibers, particles, flakes, and chips, to larger elements, such as veneers and laminates. Element morphology influences the properties of the finished panel and the element size and/or geometry may be altered by various conversion and reduction methods, such as pulping, refining and/or steaming. The processed elements are then consolidated into a panel through either a wet or a dry forming process.

Wood composites made using a “wet-forming” method are products made by dispersing the processed wood fibers in water. The use of water as the distributing medium for cellulosic materials promotes development of lignin and hydrogen bonds during heating and drying processes. If sufficient natural lignin is retained from refinement or processing of the raw material, it will behave like a thermosetting adhesive and enhance the hydrogen bonds, thereby reducing or eliminating the need to use additional binders or adhesives (Stark et al. 2010). Panels produced by wet forming processes are generally of low density and of limited strength, such as low-density cardboards, composite panel products, and agricultural fiberboards. Certain high density hardboards may be manufactured by wet processes, but these are generally of limited thickness. Medium density fiberboards suitable for, e.g., construction and furniture manufacturing, are generally not produced using a wet forming process.

Other composite wood products, such as some hardboards, medium density fiberboard (MDF), particle board (PB), and oriented strand board (OSB), are made using a “dry-forming” method in which dry wood elements are combined with resins prior to formation into panels or other forms. Manufacturing typically begins with wood reduction to produce the desired wood elements, screening to remove unwanted components, and drying the wood elements to reduce the aqueous moisture content. Typically, formaldehyde-based resins (e.g., urea-formaldehyde, “UF”) are then applied to the particles, the mixture is distributed into a mat, and the mat is then a pre-pressed and hot-pressed to consolidate the loose-formed mat into a panel. Binding of the wood elements using resins is generally faster than wet process panel production.

The relative ease-of-use, speed of processing, and lower cost of UF resins makes them very desirable for interior panel manufacturers. However, in particle board and other products manufactured using UF resins, emissions of formaldehyde gasses can present significant health concerns. There is a significant release of formaldehyde that occurs for a short time after a panel has been manufactured, followed by a secondary, continuous release that may persist for the entire life of the board (Pizzi and Mittal 2003). Phenol-formaldehyde and melamine-based resins, which are used mostly for OSB, soft plywood or flake board, also emit formaldehyde, but at lower rates than UF-based composite panels.

Alternative resins exist for use in dry-forming methods. For example, methyl diphenyl diisocyanate (MDI) is a formaldehyde-free fast-curing resin binder being used in composite wood manufacturing. However, MDI is costly and is not without health risks. For example, MDI can create a severe health risk during panel manufacturing, as it is associated with allergic reactions and sensitivities.

There remains a need for wet-process methods of producing panels that are free of toxic binders and that have sufficient density, strength, and thickness for use in manufacturing, building construction, packaging, and the like, and for panels produced by these processes.

SUMMARY OF THE INVENTION

The present invention provides methods of making binderless articles such as panels and boards, using wet forming processes. In some embodiments, the invention provides methods of forming pressed articles, including e.g., boards and panels, comprising: combining particles with cellulosic fibers in an aqueous medium to form a particle-fiber/water mixture, distributing the mixture in a form configured for removal of water from the mixture, e.g., by gravity, vacuum, cold pressing, or the like. The method comprises removing water from the mixture to produce an article having a surface and a core, pressing the article at an initial pressure and under conditions in which a fiber matrix is dewatered on a surface of the article; then pressing the article at a final pressure and under conditions in which a fiber matrix in the core of the article is cured.

The invention is not limited to a particular ratio of particles to fibers. In some embodiments, the ratio of particles to cellulosic fibers is between about 25:75 to about 75:25 (weight to weight). In some preferred embodiments, the ratio is approximately 50:50.

It is contemplated that many different kinds of particles may find use in the mixture of the present invention. In some preferred embodiments, the particles comprise wood particles, while in some embodiments, the particles comprise charcoal particles. In some embodiments, the mixture comprises wood particles and charcoal particles.

Cellulosic fibers from many sources find application in embodiments of the invention. In some preferred embodiments, the cellulosic fibers comprise paper fibers. In some embodiments, the fibers are long fibers. It is contemplated that preparations of fibers may comprise a mixture of fibers of different lengths. In some embodiments, the fibers are essentially purely long fibers, which in some embodiments, the fibers are mostly long fibers (e.g., >50%, >60%, >70%, >80%, >90%, etc). Alternatively, in some embodiments, the fibers are substantially purely short fibers, or are mostly short fibers (e.g., >50%, >60%, >70%, >80%, >90%, etc).

In certain embodiments of the methods of the invention, the article is pressed at a first pressure and a final pressure, and said first pressure is higher than said final pressure. In some particularly preferred embodiments, the first pressure is about 75 and 500 psi board pressure, and in some preferred embodiments, the final pressure is between about 10 and 50 psi board pressure.

In certain embodiments, the first pressure is done at a temperature not to exceed about 212° F., and in certain embodiments, the pressing at the final pressure is done at a temperature between about 350 and 400° F. In some embodiments, the article is subjected to a cold pressing prior to said first pressing, e.g., to remove free water from between fibers and particles.

In some embodiments, the article is pressed in three or more pressings. In certain preferred embodiments, the method comprises pressing at a second pressure after pressing at the first pressure, wherein the second pressure is lower than both the first pressure and the final pressure. In certain preferred embodiments, the first pressure is about 75 and 500 psi board pressure, and/or the second pressure is between about 10 and 50 psi board pressure, and/or the final pressure is between about 75 and 500 psi board pressure. In particularly preferred embodiments, the first pressure is done at a temperature not to exceed 212° F., and both the second and final pressures are both done at about 350 and 400° F.

In certain preferred embodiments, the article is a board or panel. In particularly preferred embodiments, the board or panel is a flat board or panel.

The present invention provides an article produced according to the methods disclosed herein. In some embodiments, the invention provides a composition comprising a binderless panel comprising wood particles and cellulosic fibers, wherein the panel is at least 0.125 inches to at least 1.0 inch thick, and wherein the panel has uniform density through the thickness of the panel. In some embodiments, the invention provides a composition comprising a binderless panel comprising wood particles and cellulosic fibers, wherein the panel is at least 0.25 inches thick, and wherein the panel has core portion disposed between a first outer portion and a second outer portion, wherein the core portion has higher density than the first outer portion and the second outer portion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a graph comparing the bending modulus of elasticity (BMOE) of a uniform density binderless panel as described herein with those of particle board (PB) and medium density fiberboard (MDF) samples. Diamond shapes represent test results from uniform core density panels according to the present invention, tested according to ASTM D1037. Triangle data-line represent ANSI 208.1 particleboard and square data-line represents ANSI 208.1 medium density fiberboard modulus of elasticity standards.

FIG. 2 shows a density profile for a typical fiberboard or particleboard showing lower density at the core of the panel.

FIG. 3 shoes a density profile for a binderless fiberboard having uniform density throughout the thickness of the panel.

FIG. 4 shows a density profile for a binderless fiberboard having higher density at the core of the panel.

FIGS. 5A and 5B show results from testing 12 samples for emissions. Twelve emission samples were extracted from the test specimens following California Specification 01350 (Cal. Dept. of Health 2004) and ASTM Standard Guide D-6007-02 (ASTM 2002) at 113, 156 and 257 hours of conditioning time (Table 1 in FIG. 5A). Formaldehyde emission factors listed in μg/m²/h were converted to parts per million (ppm) following ASTM Standard Guide E-1333-96 (ASTM 2002) based on loading factors referenced for PB and MDF (Table 2 in FIG. 5B).

DEFINITIONS

As used herein, “a” or “an” or “the” can mean one or more than one. For example, “a” surface can mean one surface or a plurality of surfaces.

“Dry-process” refers to the production of products, e.g., wood-product panels such as medium density fiberboard (MDF), particleboard (PB), and oriented strand board (OSB), by combining dried wood fibers and/or particles with resins and consolidating the material at a low moisture content (around 10%) to form rigid panels.

“Wet-process” formation of panels refers to the production of products, e.g., wood-product panels by a process in which wood products and water are processed to form a water slurry, which is then poured into a form, with water subsequently removed by vacuum, pressing and the like.

As used herein, the term “cellulosic fiber,” refers to natural and/or synthetic fibers containing cellulose. Cellulosic fiber is found, for example, in the stems, leaves, hulls, husks, and cobs of plants or leaves, branches, and wood of trees. Cellulosic material from which cellulosic fiber can be derived include virgin plant biomass and/or non-virgin plant biomass such as agricultural biomass, commercial organics, construction and demolition debris, municipal solid waste, waste paper, and yard waste, and includes trees, shrubs, grasses, wheat, wheat straw, sugar cane bagasse, corn, corn husks, corn kernel including fiber from kernels, products and by-products from milling of grains such as corn, rice, wheat, and barley (including wet milling and dry milling), as well as municipal solid waste, waste paper, and yard waste. The cellulosic material can also be, but is not limited to, herbaceous material, agricultural residues, forestry residues, and paper mill residues. Additional examples include but are not limited to branches, bushes, canes, corn and corn husks, energy crops, forests, fruits, flowers, grains, grasses, herbaceous crops, leaves, bark, needles, logs, roots, saplings, short rotation woody crops, shrubs, switch grasses, trees, vegetables, fruit peels, vines, sugar beet pulp, wheat midlings, oat hulls, hard and soft woods, organic waste materials generated from agricultural processes including farming and forestry activities, specifically including forestry wood waste, or a mixture thereof. Methods for preparing cellulosic material to produce, e.g., pulp, for use in the methods and systems of the present invention are common in the art. Numerous technologies have been developed, implemented, and improved over the last century in modern pulp and paper production. Currently, there are several main pulp production processes: (1) kraft pulping process uses sodium hydroxide and sodium sulfide to produce both unbleached and bleached pulps (after subsequent bleaching); (2) soda pulping uses sodium hydroxide to produce bleached and unbleached pulps from non-woody biomass; (3) mechanical pulping uses mechanical disk refining to produce newsprint-grade pulps, (4) chemical-mechanical pulping uses chemicals, such as sulfite and hydrogen peroxide, to pre-treat wood chips prior to disk refining to produce pulps for certain grade applications, such as boxes; and (5) sulfite pulping, including acidic to alkaline sulfite pulping, produces dissolved pulp. Neutral sulfite pulping (a special chemical-mechanical pulp, NSSC) uses sulfite with a base. Detailed description of different pulping processes can be found, e.g., in two series books: (I) “Pulp and Paper Manufacture”, Vol. 1-10, 3rd ed., 1985, Ed: the Joint text book committee of the paper industry, TAPPI, ISBN: 0-919893-04-X, and (II) “Papermaking Science and Technology”, Eds: Gullichsen and Paulapuro, Vol 1-19, Fapet Oy, 2000, ISBN: 952-5216-00-4.

As used herein, the term “long fibers” as used in reference to cellulosic fibers refers to fibers of about 0.04 to 0.24 inches, e.g., as obtained from pulping processes that produce paper from virgin or recycled materials obtained from wood or agricultural sources.

As used herein, the term “short fibers” as used in reference to cellulosic fibers refers to fibers of about 0.004 to 0.04 inches, e.g., as obtained from pulping processes that produce paper from primarily recycled paper or agricultural materials.

As used herein, the term “binderless” as used in reference to boards, panels, and other articles produced from wood elements, e.g., fibers, particles etc., refers to articles produced using a process that comprises bonding of the natural components of the wood elements, e.g., natural lignins, without addition of exogenous resins or adhesives.

As used herein the terms “cured” and “dried” are used interchangeably to refer to a condition wherein moisture is depleted, e.g., from a board or fiber matrix and the constituent elements are bonded.

As used herein, the term “aqueous medium” refers to water or a solution in which water is the primary solvent of any dissolved chemicals present.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to wet-forming processes and compositions for producing wood-based articles without the use of binders or resins. More particularly, the present invention relates to the production of wet-formed articles having improved strength, density, and thickness. In some embodiments, articles of the present invention comprise additional particles, such as charcoal, that are not suitable for use in hardboards and MDFs produced by dry-forming methods, i.e., formed using binders or resins. Embodiments of the invention are described in this Description, and in the Summary of the Invention, above, which is incorporated here by reference. Although the invention has been described in connection with certain specific embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.

The wet-forming systems of the present invention take advantage of the fact that fiber-to-particle and fiber-to-fiber bonds can be achieved without resin through natural hydrogen and lignin bonding, combined with the improved water drainage and moisture release provided by the presence of particles in a fiber matrix.

All wet-forming methods require that steps to eliminate water during pressing and curing. Difficulties in draining and expelling excess water through vacuum and/or pressing have either limited the thickness of panels that could be produced from pure fiber pulps, or have limited the pressure procedures that could be used during pressing and curing, thereby limiting the products to lower density and lower strength panels, or to thin, dense hardboard.

For example, hardboard produced from cellulosic fibers through the use of high pressure/high temperature lignin bonding has been limited to about ⅜″ in thickness. Thicker wet process boards have been produced from cellulosic fibers, e.g., recycled paper, but the thicker panels are manufactured at lower pressure, producing a very low density panel 26 to 28 lbs/ft³.

Cellulosic fibers, such as recycled paper fibers, naturally bond when wet-formed and pressed. Without pressure, these fibers produce a mat having a density of about around 6.2 to 12.5 lbs/ft³. A 100% fiber panel, pressed at low pressures, will generally form a low-strength panel because the fiber-to-fiber contact is insufficient to permit supportive bonding. A low-density mat simply does not have sufficient material to produce the high number of fiber-to-fiber contacts necessary to produce strong bonds and a strong panel. A density above 37 lbs/ft³ is needed to produce something similar to commercial particleboard or MDF products currently on the market.

The binderless panel provided herein is made from a combination of particles, e.g., wood particles, and cellulosic fibers (e.g., pulped wood and/or paper fibers), that are formed into a panel using only water, heat and pressure. The wood particles are of such size as to be the same or larger in diameter than the surrounding cellulosic fibers. The wood particles and fibers are mixed in water for even distribution and homogenization of the mixture. Once mixed and distributed into a forming box with a screen on the bottom, a vacuum is pulled below the screen to cause the mixture to start forming onto the screen. Particles and fibers intertwine as the layer continues to form. The particle-fiber interaction provides several benefits in products made from the co-mingled material:

-   -   1. Mixing wood particles with the cellulosic fibers reduces the         particle packing density and increases the drainage rate         compared to what would occur with the use of pulp fibers only,         without particles. As the two materials form an increasingly         thick layer of material, wood particles are dispersed throughout         the paper fibers. The dispersal of the wood particles opens up         passages through the formed mat for the water to flow through         the mat structure. Better water flow increases the drainage         rates and/or decreases forming time. During pressing, the         passages assist in de-watering as well. Improved de-watering         allows formation of a thicker mat within a reasonable period of         time.     -   2. The fibers entrap wood particles and inter-fiber bonding and         fiber-to-particle secures the particles in the mat and panel.         Because the fibers are much longer than the particles, the         fibers form bonds crossing over several particles, resulting in         an intertwined structure that holds the particles in place.         Under heat and pressure, the fibers bond to the particles and to         each other, resulting in a thick board of medium density.     -   3. The panels made from this process exhibit a density profile         that is significantly more uniform than that of typical         particleboard or medium density fiberboard because of the         intertwined structure of the materials and the         drying/heating/pressing process.     -   4. In some embodiments, a fiberboard having a higher density in         its core is produced by the use of a modified pressing sequence.         This contrasts with the typical structure for wet-formed         fiberboards, which are denser near the surfaces and less dense         in the core. A low density core is a configuration that is         typically seen in fiberboards used, e.g., in ready-to-assemble         furniture fabrication, in applications where screws are used for         assembly, and for fastening into the core of the board (either         through a face, or edge-on). Low-density core fiberboard does         not have significant screw-holding power, so screws can be         dislodged, weakening assembled products. Uniform density and         high density cores in fiberboard increase screw holding         strength.     -   5. When no resin binder is used, beneficial properties of         natural wood extractives (low molecular weight organic         compounds, e.g., fatty acids, resin acids, waxes and terpenes)         found naturally in the wood particles are not masked. Reclaimed         or recycled cedar wood particles, for example, may be combined         with paper fibers to make a board panel that has the aromatic         properties of cedar, for use in place of cedar, e.g., in moth         protection for stored clothing (e.g., in closets or drawers), or         in interior closed spaces that benefit from an ambient cedar         aroma, such as in spa or other therapeutic settings. Such         beneficial properties can be inhibited or masked if binding         resins having harmful off-gassing characteristics are used to         bond the wood particles and fibers.     -   6. Particles other than wood particles may also be used, alone         or in combination with wood particles. For example, charcoal         particles may be used to replace or supplement wood particles in         combination with cellulosic fiber in the manufacture of         articles. Charcoal particles find use, e.g., in the absorbance         of harmful gases or compounds in closed interior environments,         such as in hospital rooms, offices, or homes, where panels may         be used to “soak up” harmful compounds over time. In some         embodiments, panel installations may be configured such that         used panels may be replaced from time to time with fresh panels,         to provide fresh charcoal particles.

Selection of Particles and Fibers

Wood residues and pulp fibers may be selected from any number of raw and post-consumer waste sources. For example, particles and fibers may be derived from waste wood, residue from lumber processing or construction (e.g., waste wood and sawdust) and/or from farming and agriculture (e.g., crop residue, landscaping waste). The wet-processing method allows for the use of these particles in combination with cellulosic fibers, and creates minimal damage to the fibers.

In some embodiments, secondary raw wood particle processing, such as refinement and screening, may be used for improved performance or appearance. Additional processing increases utilization of otherwise non-useable material. Large pieces of wood (chunk wood), for example, may generally be considered less-than-ideal panel material. Large pieces, however, may be mechanically reduced to smaller and more homogeneous particles. These particles, in turn, allow for improved mixing with recycled paper fiber, resulting in a better-bonded board with a more uniform appearance. In certain preferred embodiments, particles greater in size than the length of the average fiber length are re-sized to smaller sizes so the fibers can best entrap the particles.

Different fiber types may be used in different process embodiments. Fibers may be pulped from virgin wood or waste wood, for example. In certain preferred embodiments, fibers are generated through processing recycled or waste paper, such as office paper or newsprint or corrugated cardboard. Fibers may be selected for properties they contribute to the process or finished article. For example, longer fibers from selected grades of paper find application in processes having faster water drainage faster and resulting in increased wet mat strength (Maloney, Thomas M. 1977. Modern Particleboard & Dryprocess Fiberboard Manufacturing San Francisco, Calif.:182; Suchsland and Woodson, supra).

Panel Formation

A mixture of particles, fibers and water is made. In certain preferred embodiments, the particles are selected as being those that pass through no. 7 mesh (2,830 micrometer; 0.111 inches) or smaller. In other embodiments, larger particles may be used. In some preferred embodiments, the ratio of particles to fibers (p:f) is approximately 50:50 (weight to weight), while in other embodiments, the ratio can vary between about 25:75 to about 75:25 by weight, depending on the properties desired in the final article. In certain preferred embodiments, the wet-forming consistency (consistency=dry fiber weight/water weight) is approximately 1.5%, but in other embodiments, the consistency may vary by up to about ±1% (e.g., from about 0.5% to about 2.5%). In certain preferred embodiments the panel is flat, but in other embodiments, non-flat (e.g., curved) sections are formed using this panel formation process.

Pressing and Curing

After vacuum formation of the wet mat or article, the mat is hot pressed. In some embodiments, the wet mat is cold pressed using standard methods to remove residual water prior to hot pressing and drying.

Uniform Core Density Panels

In some preferred embodiments, the article is first pressed between two flat, hot platens at about 75 to 500 psi board pressure (e.g., at 200 psi board pressure), at temperature not to exceed 212° F., and held for 5 to 30 seconds to squeeze out as much water as possible. The pressure is released, then the board is pressed at a temperature of about 350 to 400° F. (177-204° C.), according to a single or multi-linear thickness schedule to achieve the final thickness when the board is dry. The second press cycle includes either a constant pressure process of 10 to 50 psi board pressure, or as a thickness profile that follows board moisture content as it leaves the board. The board thickness decreases while the moisture leaves the board. The closure rate follows the thickness. This produces a constant board density through the thickness of the board.

High Core Density Panels

In some embodiments, an alternative pressing process is used to reverse the density profile from low density in the core to a high-density core product. As with the procedure above, the article is first pressed between two flat, hot platens at about 75 to 500 psi board pressure (e.g., at 200 psi board pressure) at temperature not to exceed 212° F., and held for 5 to 30 seconds to squeeze out as much water as possible. The board is then pressed at low pressure (10 to 50 psi board pressure, at 350 to 400° F. (177-204° C.)) for sufficient length of time to cure or dry the fiber matrix on the outside surface of the board, followed by higher pressure (75 to 500 psi board pressure) before the board is completely dry or cured, and held for a length of time to cause the center of the board to cure or dry under high pressure. For a ⅝″ board, the high pressure is applied for about 5 to 30 minutes, but the time may be adjusted, depending on the density and final thickness of the board, and on the particular mixture of particles and fibers.

Standard methods of producing PB, MDF, and hardboard also follow a process where high pressure is applied for 30 to 120 seconds followed by significant reduction in pressure to de-gas the board of steam pressure, followed by higher pressure at the end of the pressing cycle (High-low-high). However, both the rationale for the high-low-high pressing process and the outcome in board density are very different. A primary goal of prior art processes is to produce boards as quickly as possible. To facilitate this, the process comprises an intermediate step of reducing the pressure in order to de-gas and to release moisture. The high pressure applied at the end of the pressing sequence does not increase the density in the core of the board because the board's residual moisture content is no longer sufficient to cause bonding. In contrast, in the modified press sequence of the present invention, the increase in pressure at the end of the pressing sequence is done when the center of the board is moist, and has the effect of bringing the moist fibers together to initiate improved fiber-to-fiber bonding, thereby increasing the density in the core of the panel. Little increase in density occurs in the outer face portions that were dried during the first phase of the pressing, so the process yields a board with higher density in the core than at the faces.

In certain embodiments, screens are used on both sides of the board during pressing to allow the panels to release moisture through the both faces, providing a more uniform process on top and bottom. Further processing may include post heat treatment for further natural enhancements of performance properties.

EXPERIMENTAL EXAMPLE Panel Fabrication

Test panels were wet-formed from 50% poplar sawdust and 50% paper pulp. The particles used in the mixture were passable through no. 7 mesh. The ratio of particles to fibers (p:f) was approximately 50:50, and the wet-forming consistency was approximately 1.5%. The mixture was spread in a form comprising a bottom screen, and most of the free water was removed by vacuum.

Wet mats were placed between two flat cold platens and pressed at a temperature <212° F., with pressure increasing up to 200 psi board pressure within 120 seconds, and held at pressure for an additional 5 to 30 seconds to squeeze out more water. Pressure was released, and the panel was then further hot-pressed at 350 to 400° F. (177-204° C.) using a single or multi-linear thickness schedule.

The board shown to have a uniform density was obtained using position control during hot-pressing to go from 1.5 inches to 1.25 inches in 120 seconds; 1.25 inches to 0.75 inches in 600 seconds; from 0.75 inches to 0.625 inches in another 600 seconds; followed by additional drying time of 600 seconds held at 0.625 inches. Modifications to the time and rates may be adjusted so that the pressure during the drying is done with less than 50 psi board pressure.

For high core density panels, the particle/fiber slurry was prepared and formed as described above. The board was hot-pressed using position control to go from 1.5 inches to 1.25 inches in 120 seconds; 1.25 inches to 0.75 inches in 600 seconds; from 0.75 inches to 0.625 inches in another 600 seconds; followed by high-pressure of 200 psi board pressure at an additional drying time of 600 seconds. Modifications to the time and rates may be adjusted so that the pressure during the initial drying is done with less than 50 psi board pressure. The high pressure at the end creates the high density core.

Testing

Test specimens were cut from the flat panels to evaluate bending properties as outlined in ASTM Standard D 1037 (ASTM 1996). Twenty-five specimens were conditioned in a 50% relative humidity (RH), 72F conditioning room. Static bending tests were performed and the modulus of elasticity (MOE) was calculated (see ASTM Designation: D 1037-06a (ASTM 2006)). Comparison properties are ANSI 208.1 Standard property values for PB and MDF.

FIG. 1 compares the bending modulus of elasticity (BMOE) of the uniform density test board with the particle board (PB) and medium density fiberboard (MDF) ANSI 208.1

Emissions Testing

Four test specimens were cut from panels to extract and analyze their emissions at the Indoor Environment Department, Environmental Energy Technologies Division of the Lawrence Berkeley National Labs. Twelve emission samples were extracted from the test specimens following California Specification 01350 (Cal. Dept. of Health 2004) and ASTM Standard Guide D-6007-02 (ASTM 2002) at 113, 156 and 257 hours of conditioning time (refer to Table 1 in FIG. 5A). Formaldehyde emission factors listed in μg/m²/h were then converted to parts per million (ppm) following ASTM Standard Guide E-1333-96 (ASTM 2002) based on loading factors referenced for PB and MDF (refer to Table 2 in FIG. 5B).

The maximum allowable emissions, as stated in the Formaldehyde Standards for Composite Wood Products Act, for Particleboard (PB) range from 0.09-0.18 ppm and for Medium Density Fiberboard (MDF) range from 0.11-0.21 ppm. Formaldehyde emissions analysis of the results from these specimens shows that these emissions factors for the tested samples are significantly lower than those for either PB or MDF. This can be calculated by starting with Equation (5) in the ASTM-E1333-96 (2002) document (see below) and solving for the steady state concentration (Cs), then using the experimental parameters specified in the standard to convert the emission factor (EF) to concentration (Cs).

Different materials use different specified loading factors that reference the conversion. For PB flooring or industrial PB, the loading factor=0.43 m2/m3 and for MDF the loading factor=0.26 m2/m3. Using the appropriate reference loading factor (L), one can use the equation derived from Equation 5 in the ASTM-E1333-96 (2002) to estimate the steady state concentrations in ppm. The panels of these embodiments of the present invention show significantly lower ppm concentration values than the standard limits as shown in Table 2, (FIG. 5B).

Physical Panel Properties

For boards produced using standard and modified press sequences, density profiles were determined and compared to the density profile for standard MDF. FIG. 2 shows a typical density profile through the thickness for PB or MDF representing a common high-low-high density core panel. FIG. 3 shows the uniform density profile achieved using the particle-fiber mixture of the present invention. FIG. 4 shows a higher core density profile at the mid-point of thickness that is obtained using the particle-fiber mixture and the modified press sequence of the present invention.

When using 100% recycled newsprint fibers (as are used to make paper), drainage to form a ⅝″ thick mat takes 10 minutes or longer. Use of the mixture of wood particles and fibers decreased drainage time by about 50%. Drainage times will vary depending on the particle geometry and fiber-to-particle weight distribution.

These data showed that:

-   -   Medium density binderless fiberboards can be made from a         combination of pulp fiber and particles. The particles improve         water drainage compared to fiber-only slurries, allowing         production of thicker panels than can be formed by wet-forming         from fiber alone, without loss of the strength that comes from         fiber-to-fiber bonding at high temperatures and pressures.     -   A modified pressing sequence yields binderless fiberboard with         high density core.

BIBLIOGRAPHY

-   ASTM International (2002) “Standard Test Method for Determining     Formaldehyde Concentrations in Air and Emission Rates from Wood     Products Using a Large Chamber”. Designation: E 1333-96 (Reapproved     2002) -   ASTM International (2002) “Standard Test Method for Determining     Formaldehyde Concentration in Air from Wood Products Using a Small     Scale Chamber”. Designation:D 6007-02 -   ASTM International (2006) “Standard Test Methods for Evaluating     Properties of Wood Base Fiber and Particle Panel Materials”.     Designation: D1037-06a -   California Department of Health Services (2004) “Standard Practice     for the Testing of Volatile Organic Emissions from Various Sources     Using Small-Scale Environmental Chambers”. Environmental Health     Laboratory Branch. Jul. 15, 2004 -   Maloney, Thomas M. 1977. Modern Particleboard & Dryprocess     Fiberboard Manufacturing San Francisco, Calif.:182 -   Pizzi, Antonio and K. L. Mittal. 2003. Handbook of adhesive     technology:646 -   Siegel, David M.; Frankos, Vasilios H. and Marvin A.     Schneiderman. 1983. Formaldehyde Risk Assessment for Occupationally     Exposed Workers. Abstract:355 -   Stark, Nicole M.; Cai, Zhiyong; Carll, Charlie G. 2010. Wood-Based     Composite Materials-Panel Products-Glued-Laminated Timber,     Structural Composite Lumber, and Wood-Nonwood Composite Materials.     Wood Handbook, Wood as an Engineering Material. General Technical     Report FPL-GTR-190. Madison, Wis.: U.S. Department of Agriculture,     Forest Service, Forest Products Laboratory: Chapter 11: 11-1-11-28. -   Suchsland, O. and, G. E Woodson. 1986. Fiberboard manufacturing     practices in the United States. U.S. Dept. of Agriculture, Forest     Service. -   Zhang, Luoping; Freeman, Laura E. Beane; Nakamura, Jun; Hecht,     Stephen S.; Vandenberg, John J.; Smith, Martyn T.; and Babasaheb R.     Sonawane. 2010. Formaldehyde and Leukemia: Epidemiology, Potential     Mechanism, and Implications for Risk Assessment. Environmental and     Molecular Mutagenesis. Review Article. 51:181-191

All publications, patents and patent applications referenced above are incorporated herein by reference in their entireties, for all purposes. The description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein can be applied to other embodiments without departing from the spirit or scope of the invention. Thus, it is to be understood that the description and drawings presented herein represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the present invention is accordingly limited by nothing other than the appended claims. 

1. A method of forming a pressed article, comprising: i) combining particles with cellulosic fibers in an aqueous medium to form a particle-fiber/water mixture; ii) distributing said mixture in a form, wherein said form is configured for removal of water from said mixture; iii) removing water from said mixture to produce an article having a surface and a core; iv) pressing said article at an initial pressure, under conditions wherein a fiber matrix is dewatered on a surface of said article; and v) pressing said article at a final pressure under conditions wherein a fiber matrix in said core of said article is cured.
 2. The method of claim 1, wherein step iii comprises applying a vacuum to said form to remove water from said mixture.
 3. The method of claim 1, wherein the ratio of particles to cellulosic fibers is between about 25:75 to 75:25 (weight to weight).
 4. The method of claim 1, wherein said particles comprise wood particles and/or charcoal particles.
 5. The method of claim 1, wherein said cellulosic fibers comprise paper fibers and/or agricultural fibers.
 6. The method of claim 1, wherein said cellulosic fibers comprise long fibers and/or short fibers.
 7. The method of claim 1, wherein said first pressure is higher than said final pressure.
 8. The method of claim 1, wherein said first pressure is about 75 and 500 psi board pressure.
 9. The method of claim 1, wherein said final pressure is between 10 and 50 psi board pressure.
 10. The method of claim 1, wherein said pressing at said first pressure is done at a temperature not to exceed 212° F.
 11. The method of claim 1, wherein said pressing at said final pressure is done at a temperature between 350 and 400° F.
 12. The method of claim 1, further comprising pressing at a second pressure after pressing at said first pressure, wherein said second pressure is lower than said first pressure or said final pressure.
 13. The method of claim 12, wherein said first pressure is between about 75 and 500 psi board pressure; said second pressure is between about 10 and 50 psi board pressure; and/or said final pressure is between about 75 and 500 psi board pressure.
 14. The method of claim 12, wherein said pressing at said first pressure is done at a temperature not to exceed 212° F.
 15. The method of claim 12, wherein said pressing at said second pressure is done at 350 and 400° F.
 16. The method of claim 12, wherein said pressing at said final pressure is done at a temperature between 350 and 400° F.
 17. The method of claim 1, wherein said article is a panel.
 18. The method of claim 17, wherein said panel is a flat panel or a curved panel.
 19. A composition comprising a binderless panel comprising wood particles and cellulosic fibers, wherein said panel is at least 0.125 inches to 1.0 inch thick or greater, and wherein said panel has uniform density through the thickness of the panel.
 20. A composition comprising a binderless panel comprising wood particles and cellulosic fibers, wherein said panel is at least 0.125 inches thick, and wherein said panel has core portion disposed between a first outer portion and a second outer portion, wherein said core portion has higher density than said first outer portion and said second outer portion. 